1. Field of the Invention
The present invention generally relates to planar display devices and more particularly to a liquid crystal display device having a planar light source wherein the luminance is improved.
2. Description of the Related Art
In the planar display devices, particularly the liquid crystal display panels, a backlight illumination unit is provided behind a liquid crystal cell panel for improving the visibility of the displayed image. In correspondence to the planar construction of the liquid crystal display unit, the backlight illumination unit has also a planar construction.
FIG. 1 shows an example of the construction of a liquid display panel.
Referring to FIG. 1, the liquid crystal display panel includes a liquid crystal cell panel 1 having a display area 2, wherein a display pattern is formed on the display area 2 by inducing an orientation in the liquid crystal molecules in the panel 1. The liquid crystal display device further includes a planar illumination unit 3 behind the liquid crystal cell panel 1, and the planar illumination unit 3 radiates light at a luminous surface 4 that faces the liquid crystal cell panel 1. In such a liquid crystal display panel, the planar illumination unit 3 is required to have a high and uniform luminance throughout the luminous surface 4.
FIG. 2 shows the schematical construction of the planar illumination unit 3 used in the conventional liquid crystal display device.
Referring to FIG. 2, the unit 3 includes an optical guide plate 5 of a transparent material such as an acrylic resin, and a cold cathode tube 6 is disposed so as to face a side wall 5a of the optical guide plate 5. Thereby, the optical guide plate 5 guides the light that has been produced by the cold cathode tube 6 and injected at the side wall 5a and emits the same from a principal surface 11 that acts as a luminous surface.
The cold cathode tube 6 is held by a holder 7 in alignment with the optical guide plate 5, wherein the holder 7 has a reflective surface 7a for reflecting the light that has been produced by the cold cathode tube 6 to the foregoing side wall 5a of the waveguide plate 5. In addition, there is provided a planar reflector 8 at a side wall 5b opposite to the side wall 5a for reflecting back the light that has been emitted from the optical guide plate 5. Further, a planar reflector 10 is provided adjacent to a bottom principal surface 9 of the optical guide plate 5 such that the reflector 10 faces the surface 9. Thereby, the light that has leaked from the optical guide plate 5 through the bottom surface 9 is reflected back to the optical guide plate 5.
Above the optical guide plate 5, there is provided an optical scattering plate 12 such that the plate 12 faces the foregoing luminous surface 11. The optical scattering plate 12 has a milky whitish appearance and causes a scattering in the light that passes therethrough. As a result, the backlight emitted from the optical scattering plate 12 has a uniform luminance.
In such a liquid crystal display panel, it is necessary to increase the luminance of the backlight that is emitted at the luminous surface 11 of the optical guide plate 5. On the other hand, experience indicates that mere increase of the optical power of the cold cathode tube 6 is not effective for the desired increase of luminance of the planar illumination unit 3. Contrary, such an increase of the optical power of the cold cathode tube 6 invites an increased electrical power consumption and increased size of the device. In view of the fact that the liquid display panels are widely used in small, portable computers and word processors, such an increase in the electrical power or size is not acceptable.
In designing the planar illumination unit 3, therefore, it is important to reduce the loss of the light that has been emitted by the cold cathode tube 6 and to maximize the proportion of the light that is emitted from the luminous surface 11. The inventors of the present invention have conducted a series of analysis with respect to the propagation of the light that has been emitted from the cold cathode tube 6 and discovered the following facts. The result of the investigation is reported in MESAKI, Y., et al., TECHNICAL REPORT OF IEICE, EID92-126, ED92-159 (1993-02), pp.83-90, MESAKI, Y., et al., TECHNICAL REPORT OF IEICE, EID93-33 (1993-07), pp. 45-50, MESAKI, Y., et al., TECHNICAL REPORT OF IEICE, CPM93-33, OME93-19 (1993-07), all of which are published after the priority date of the present application.
The light emitted from the cold cathode tube 6 propagates typically along one of the paths P.sub.1 -P.sub.5 indicated in FIG.2, wherein P.sub.1 represents the path of the light that enters directly into the optical guide plate 5 through the side wall 5a; P.sub.2 represents the path of the light that enters the optical guide plate 5 after a reflection at the holder 7; P.sub.3 represents the path of the light that is absorbed by the holder 7; P.sub.4 represents the path of the light that is absorbed by the cold cathode tube 6 after reflection at the holder 7; and P.sub.5 represents the light that has been emitted from the cold cathode tube 6 but failed to enter the optical guide plate 5. In correspondence to the paths P.sub.1 -P.sub.5, there holds a relationship EQU LP.sub.1 +LP.sub.2 +LP.sub.3 +LP.sub.4 +LP.sub.5 =LP.sub.0,(1)
wherein LP.sub.1 -LP.sub.5 represents the amount or energy of the light for each of the paths P.sub.1 -P.sub.5 while LP.sub.0 represents the total energy of the light that has been emitted from the cold cathode tube 6.
It should be noted that the above analysis indicates that the energy of the light that enters actually to the optical guide plate 5 is represented as EQU LP.sub.in =LP.sub.1 +LP.sub.2 ( 2)
where LP.sub.in represents the energy of the light entering into the waveguide plate 5. The rest of the light has been lost. Thus, conventional planar illumination unit 3 suffers from the problem that the efficiency of injecting the light from the cold cathode tube 6 to the optical guide plate 5 is poor when the contribution of the terms LP.sub.3 -LP.sub.5 is substantial. With this regard, one should note that the cold cathode tube 6 carries a fluorescent material for emitting the light, while such a fluorescent material also absorbs light that is incident thereto. Although the light absorbed by the fluorescent material may be re-emitted, substantial loss of light cannot be avoided during such a process.
Meanwhile, there is a proposal to align the direction of emission of the light from the planar illumination unit 3 so as to increase the luminance of the unit 3 with respect to the direction perpendicular to the unit 3. For this purpose, a prismatic lens film carrying, on a transparent film, a number of parallel ridges or stripes of triangular cross section, is disposed on the scattering plate 12. However, such a prismatic lens film tends to cause an excessively sharp directivity in that the luminance drops sharply when the view angle has exceeded a certain threshold. When such a light source unit 3 is used for a liquid crystal display device, therefore, there occurs an inconvenience that the person who is not facing the display panel directly cannot see the displayed image at all. It is desirable to provide a more gentle change of the luminance as a function of the view angle.